1. Field of the Invention
The present invention relates to post-tension anchor systems. More particularly, the present invention relates to dead-end anchors used in such post-tension systems. More particularly still, the present invention relates to devices used for retaining a tendon within the dead-end anchor of a post-tension system. Additionally, the present invention relates to devices for preventing the shrinkage of a sheathing that extends over the tendon.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, concrete design began to evolve. Concrete has the advantages of costing less than steel, of not requiring fireproofing, and of having plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economical and popular. Reinforced-concrete framing is seemingly a simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, comprising a mixture of water, cement, sand, and stone or aggregate and having proportions calculated to produce the required strength, is set, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as five hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing tendons of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
In typical post-tension systems, the tendon is received between a pair of anchors. One of the anchors is known as the “live-end” anchor, and the opposite end is known as the “dead-end” anchor. The “live-end” anchor receives the end of the tendon which is to be tensioned. The “dead-end” anchor holds the tendon in place during the tensioning operation. Under typical operations, a plurality of wedges are inserted into an interior passageway of the anchor and around the exterior surface of the tendon. The tendon is then tensioned so as to draw the wedges inwardly into the interior passageway so as establish compressive and locking contact with an exterior surface of the tendon. This dead-end anchor can then be shipped, along with the tendon, for use at the job site.
One technique for forming such dead-end anchors is to insert the end of a tendon into the cavity of the anchor, inserting wedges into the space between the tendon and the wall of the cavity and then applying a tension force onto another end of the tendon so as to draw the wedges and the end of the tendon into the cavity in interference-fit relationship therewith. This procedure is somewhat difficult since the tendon can have a considerable length and since the use of tension forces can create a somewhat unreliable connection between the wedges and the tendon. Experimentation has found that the application of compressive force onto the end of the tendon creates a better interference-fit relationship between the wedges, the end of the tendon and the wall of the cavity of the anchor.
Another technique is described in U.S. Pat. No. 6,513,287, issued on Feb. 4, 2003 to the present inventor. This patent describes a method and apparatus for forming an anchorage of a post-tension system in which a tendon is positioned within a cavity of the anchor such that an end of the tendon extends outwardly of the cavity. A plurality of wedges are mechanically inserted within the cavity between the tendon and a wall of the cavity. Pressure is applied to an end of the tendon such that the tendon and the wedges are in interference-fit relationship within the cavity. A compression mechanism is used having a cylindrical member and a plunger extending in a channel of the cylindrical member. The wedges are attached to the cylindrical member and the cylindrical member is moved toward the cavity such that the wedges enter a space between the tendon and the wall of the cavity. The plunger applies a compressive force to the end of the tendon when the end of the tendon is in the channel of the cylindrical member.
One of the problems with conventional dead-end anchorages is that the sheathing over the tendon has a tendency to shrink over time. The shrinkage is the result of various factors. One major factor is that the sheathing is formed over the tendon in an extrusion process. As such, the polymeric material used for the sheathing is relatively hot as it exits the extrusion process. Immediately after leaving the extrusion process, the tendon, along with the sheathing, is tightly wound around a spool. During shipment, the tight winding of the tendon around the spool will mechanically resist any shrinking of the sheathing over the lubricated exterior of the steel cable on the interior of the tendon. When the cable is unwound from the spool, these mechanical forces are released. As such, as the tendon is installed in an anchor, the relaxation of these mechanical forces will generally and slowly cause the sheathing to shrink over the length of the tendon. After the tendon is connected to a dead end anchorage, the end of the sheathing will tend to the shrink slowly away from the dead end anchorage.
The problem that affects many anchorage systems is the inability to effectively prevent liquid intrusion into this area of the unsheathed portion. In normal practice, a liquid-tight tubular member is placed onto an end of the tendon so as to cover an unsheathed portion of the tendon. The tubular member slides onto and over the trumpet portion of the encapsulated anchor so as to be frictionally engaged with the trumpet portion of the anchor. The opposite end of the tubular member will include a seal that establishes a generally liquid-tight connection with the sheathed portion of the tendon. Corrosion-protection tube technology suggests placement of an additional tube over the polymeric encapsulation of the tendon. In certain circumstances, these tubes are sometimes improperly installed and are simply additional components that need to be associated with the post-tension system. As such, it adds additional costs and can require additional labor associated with the installation of the tube. Thus, a need has developed so as to avoid the use of corrosion-protection tubes with the dead-end anchor of a post-tension anchor system.
Various patents have been filed by the present inventor addressing the need to avoid the use of corrosion-protection tubes. For example, U.S. patent application Ser. No. 11/861,197, filed on Sep. 25, 2007, discloses a sheathing-retaining article for use with a post-tension anchorage system that has a wedge with a tendon-retaining portion and a sheathing-retaining portion. The tendon-retaining portion has a channel extending longitudinally therealong. The channel is suitable for retaining the tendon therein. The tendon-retaining portion has a tapering outer surface with a wide end at one end of the wedge and a narrow end spaced therefrom. The sheathing-retaining portion extends outwardly from the narrow end of the tendon-retaining portion. The sheathing-retaining portion engages a sheathing of a tendon extending through the channel of the wedge.
The above application added a sheathing lock device to the wedges of a dead-end anchorage system. Wedges that are used in dead-end anchorages have conventionally secured only the unsheathed portion of the tendon within the cavity of the anchor. These wedges have been used so as to impart a frictional engagement, under high pressures, against the outer surface of the unsheathed portion of the tendon. A sheathed portion of the tendon also extends within the cavity of the anchor. It is the sheathing of the sheathed portion within the cavity that tends to shrink without the sheathing lock design of the above application.
It was found, through experimentation, that making a wedge with a sheathing lock on the end thereof posed a challenge of positioning both the wedge and the sheathing lock optimally within the cavity of the anchor. Thus, a need arose for creating a separate sheathing lock to fit within the cavity of the anchor along with the wedges that fix the unsheathed portion of the tendon within the cavity.
In the past, the present inventor has filed various patent applications addressing this problem. For example, U.S. patent application Ser. No. 11/874,087, filed on Oct. 17, 2007, discloses an apparatus for preventing shrinkage of a sheathing of a tendon that has an anchor body having a cavity formed in an interior thereof, a tendon extending into the cavity, a fixing element engaged with the sheathing for fixing a position of the sheathing on the tendon, and a pair of wedges in frictional engagement with the unsheathed portion of tendon in the cavity of the anchor body. The fixing element is positioned away from the cavity of the anchor. An encapsulation is formed over the anchor body so as to define a trumpet extending outwardly from one side of the anchor body. A clamp is engaged with the sheathed portion of the tendon within the trumpet.
U.S. patent application Ser. No. 11/933,041, filed on Oct. 31, 2007, discloses an apparatus for fixing the sheathing of an end of a tendon within an anchor body of a post-tension anchor system that has an anchor body with a cavity formed in an interior thereof, a tendon extending into the cavity that has a sheathing extending at least partially thereover and has a sheathed portion and an unsheathed portion, a pair of wedges in frictional engagement with the unsheathed portion of the tendon in the cavity of the anchor body, and at least one wedge member engaged with the sheathed portion. The wedge member has a first portion and a second portion. The first portion is of a constant thickness and has an end adjacent the pair of wedges. The second portion has a first end and a second end, the second portion being of a decreasing thickness from the first end to the second end.
U.S. patent application Ser. No. 11/933,029, filed on Oct. 31, 2007, discloses a device for fixing the sheathing of an end of a tendon within an anchor body of a post-tension anchor system that has an anchor body having a cavity formed in an interior thereof, a tendon extending into the cavity having a sheathing extending at least partially thereover and having a sheathed portion and an unsheathed portion, a pair of wedges in frictional engagement with the unsheathed portion of the tendon in the cavity of the anchor body, and at least one wedge member engaged with the sheathed portion. The wedge member has a wide end and a narrow end, the wide end being adjacent to the pair of wedges. The wedge member has a decreasing thickness from the wide end to the narrow end.
U.S. patent application Ser. No. 11/950,295, filed on Dec. 4, 2007, discloses an apparatus for fixing the sheathing of an end of a tendon within an anchor body of a post-tension anchor system that has an anchor body having a cavity formed in an interior thereof, a tendon extending into the cavity and having a sheathing extending at least partially thereover and having a sheathed portion and an unsheathed portion, a pair of wedges engaged with the unsheathed portion of the tendon in the cavity of the anchor body, and a wedge member engaged with the sheathing of the sheathed portion. The wedge member is a unitary piece having a longitudinal split extending from an end of the piece to an opposite end of the piece. The wedge member substantially encircles an interior or an exterior of the sheathing of the sheathed portion of the tendon.
Oddly, during experimentation, it was found that prior art wedges as used with the sheathing locks with the above patent applications did not properly engage the wedge members, or sheathing locks, and also had problems aligning the sheathing of the tendon within the cavity for proper engagement with the sheathing locks. Thus, a need has developed for a wedge design that both properly engages sheathing locks within the cavity of the anchor and properly aligns the sheathing of the sheathed portion of a tendon within the cavity of the anchor for proper engagement with the sheathing locks.
It is an object of the present invention to provide a wedge that engages a tendon within the cavity of an anchor.
It is another object of the present invention to provide a wedge that properly aligns the sheathing of the sheathed portion of a tendon within the cavity of an anchor.
It is another object of the present invention to provide a wedge that properly engages a sheathing lock within the cavity of an anchor.
It is still another object of the present invention to provide a wedge that aids in preventing the shrinkage of a sheathed portion of a tendon within that cavity of an anchor.
It is another object of the present invention to provide a wedge that is simple and cost effective.
It is another object of the present invention to provide a wedge that can withstand high pressures of engagement with a tendon inside the cavity of an anchor.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.